RF-link margin measurement method and system

Abstract

A system and method for conducting dynamic RF-link margin tests in an electronic toll collection for non-stationary vehicles travelling up to highway speed. The system maintains a list of candidate transponders and, during the course of a normal toll collection, if a transponder is on the list then the system schedules a margin test for a later handshake in the capture zone. At a location at or near the peak margin test the system conducts a margin test during one or more of the reader-transponder handshakes. The attenuation may be dynamically controlled by the system using a variable attenuator in the RF path from the reader RF module to the antenna. Distributing the margin measurement over multiple passes of a vehicle through the capture zone provides for a highway speed method of measuring link margin.

What is claimed is: 
     
       1. A method of testing RF margin in an electronic toll collection system having a capture zone, the method comprising:
 building a set of stored identifiers for candidate transponders that visit the capture zone multiple times; 
 detecting a transponder within the capture zone by receiving a response signal from the transponder that includes a transponder identifier; 
 determining that the transponder is a candidate transponder by comparing the transponder identifier to the set of stored identifiers for candidate transponders for margin testing; 
 if the transponder is a candidate transponder, conducting a margin test while the transponder is within the capture zone including sending a polling signal attenuated by a specified amount to the transponder, noting whether the transponder responds thereto, and storing a test result in association with said transponder identifier; 
 wherein the amount of attenuation is varied over at least two visits of the same transponder through the capture zone, so as to identify the attenuation at which a margin test fails, and, thus, the RF margin. 
 
     
     
       2. The method of  claim 1 , wherein the electronic toll collection system includes a reader and an antenna, and wherein conducting the margin test includes adding attenuation to an RF link between the reader and the antenna. 
     
     
       3. The method of  claim 2 , further comprising retrieving previous margin test information associated with the transponder identifier, wherein adding attenuation includes determining the attenuation level based upon the previous margin test information. 
     
     
       4. The method of  claim 3 , wherein the previous margin test information includes a last attenuation level during a most recent successful margin test, wherein determining the attenuation level includes increasing the last attenuation level by a step size. 
     
     
       5. The method of  claim 2 , wherein adding attenuation includes dynamically attenuating a polling signal in accordance with a predefined margin testing schedule. 
     
     
       6. The method of  claim 1 , wherein conducting the margin test includes scheduling the margin test for a selected reader-transponder handshake within the capture zone. 
     
     
       7. The method of  claim 6 , further comprising selecting the selected reader-transponder handshake based upon an estimated vehicle speed and a predetermined peak margin location. 
     
     
       8. The method of  claim 7 , further comprising determining the predetermined peak margin location based upon a previous transponder visit to the capture zone, wherein determining the predetermined peak margin location includes measuring received signal strength for each response signal for a series of response signals in the capture zone from the previous transponder; and identifying the peak margin location based upon the handshake corresponding to the strongest received signal strength measurement. 
     
     
       9. The method of  claim 1 , wherein detecting the transponder includes transmitting the polling signal in reply to which the response signal is received. 
     
     
       10. The method of  claim 9 , wherein storing the test result includes storing data indicating whether the reply is received. 
     
     
       11. The method of  claim 10 , wherein when the reply is not received and wherein, as a result, storing the test result includes storing a peak margin value based upon an attenuation level used in a most-recent successful margin test. 
     
     
       12. The method of  claim 11 , further comprising calculating an average RF margin for the capture zone based upon an average of stored peak margin values collected over a period of time. 
     
     
       13. The method claimed in  claim 1 , wherein building the set includes detecting transponders over a period of time and filtering out transponders that are detected less than a threshold number of times during the period of time. 
     
     
       14. The method claimed in  claim 1 , wherein building the set includes detecting transponders over a period of time and determining an average number of handshakes per visit, and filtering out a transponder with a handshake count more than a threshold amount different from the average number. 
     
     
       15. An electronic toll collection system, including a reader and an antenna defining a capture zone in a roadway, wherein the reader is configured to:
 build a set of stored identifiers for candidate transponders that visit the capture zone multiple times; 
 detect a transponder within the capture zone by receiving a response signal from the transponder that includes a transponder identifier; 
 determine that the transponder is a candidate transponder by comparing the transponder identifier to a set of stored identifiers for candidate transponders for margin testing; and 
 if the transponder is a candidate transponder, conduct a margin test while the transponder is within the capture zone including sending a polling signal attenuated by a specified amount to the transponder, noting whether the transponder responds thereto, and storing a test result in association with said transponder identifier; 
 wherein the reader is configured to vary the amount of attenuation over at least two visits of the same transponder through the capture zone so as to identify an attenuation level at which a margin test fails, and thereby determine an RF (radio frequency) link margin. 
 
     
     
       16. The system of  claim 15 , wherein the system further includes a variable attenuator in an RF link between the reader and the antenna, and wherein the reader is configured to conduct the margin test by adding attenuation using the variable attenuator. 
     
     
       17. The system of  claim 16 , wherein the reader includes a memory storing previous margin test information associated with the transponder identifier, and wherein the reader is configured to determine an attenuation level based upon the previous margin test information. 
     
     
       18. The system of  claim 17 , wherein the previous margin test information includes a last attenuation level during a most recent successful margin test, and wherein the reader is configured to determine the attenuation level by increasing the last attenuation level by a step size of the variable attenuator. 
     
     
       19. The system of  claim 16 , wherein the reader is further configured to add attenuation to attenuate a polling signal in accordance with a predefined margin testing schedule. 
     
     
       20. The system of  claim 15 , wherein the reader is further configured to conduct the margin test by scheduling the margin test for a selected reader-transponder handshake within the capture zone. 
     
     
       21. The system of  claim 20 , wherein the reader is further configured to select the selected reader-transponder handshake based upon an estimated vehicle speed and a predetermined peak margin location. 
     
     
       22. The system of  claim 21 , wherein the reader is further configured to determine the predetermined peak margin location based upon a previous transponder visit to the capture zone, and wherein determining the predetermined peak margin location includes measuring received signal strength for each response signal for a series of response signals in the capture zone from the previous transponder, and identifying the peak margin location based upon the handshake corresponding to the strongest received signal strength measurement. 
     
     
       23. The system of  claim 15 , wherein the reader is further configured to detect the transponder by transmitting a polling signal in reply to which the response signal is received, and wherein the reader is further configured to conduct a margin test by transmitting an attenuated polling signal and determining whether a reply is received. 
     
     
       24. The system of  claim 23 , wherein the reader includes memory and the reader is further configured to store the test result by storing data in memory indicating whether the reply is received. 
     
     
       25. The system of  claim 24 , wherein when the reply is not received the reader is further configured to store a peak margin value based upon an attenuation level used in a most-recent successful margin test. 
     
     
       26. The system of  claim 25 , wherein the reader is further configured to calculated an average RF margin for the capture zone based upon an average of stored peak margin values collected over a period of time. 
     
     
       27. The system of  claim 15 , wherein the reader is further configured to generate the set of stored identifiers for candidate transponders. 
     
     
       28. The system of  claim 27 , wherein the reader is further configured to generate the set by detecting transponders over a period of time and filtering out transponders that are detected less than a threshold number of times during the period of time. 
     
     
       29. The system of  claim 27 , wherein the reader is further configured to generate the set by detecting transponders over a period of time, and counting an average number of handshakes per visit, and filtering out a transponder with a handshake count more than a threshold amount different from the average number.